1. Field of the Invention
The invention relates to flash memories, and more particularly to data writing of flash memories.
2. Description of the Related Art
Flash memories are divided into single level cell (SLC) flash memories, multi level cell (MLC) flash memories, and triple level cell (TLC) flash memories. A memory cell of an SLC flash memory stores a single data bit. On the other hand, a memory cell of an MLC flash memory stores two data bits, and a memory cell of a TLC flash memory stores three data bits. If an SLC flash memory has the same number of memory cells with those of an MLC flash memory and a TLC flash memory, the data capacity of the MLC flash memory is double that of the SLC flash memory, and the data capacity of the TLC flash memory is triple of that of the SLC flash memory.
A memory cell of an MLC flash memory stores two data bits, wherein one of the data bits is a least significant bit (LSB), and the other of the data bits is a most significant bit (MSB). A plurality of pages of the MLC flash memory are therefore divided into a plurality of pair pages, and each pair page comprises a strong page and a corresponding weak page, wherein the strong page comprises the least significant bits of a set of memory cells, and the weak page comprises the most significant bits of the same set of memory cells. Because the strong page and the weak page of a pair page store data in a single set of memory cells, the data content stored in the strong page affects the data content stored in the corresponding weak page, and the data content stored in the weak page affects the data content stored in the corresponding strong page.
A memory cell of a TLC flash memory stores three data bits, wherein one of the data bits is a least significant bit (LSB), another of the data bits is a center significant bit (CSB), and the other of the data bits is a most significant bit (MSB). A plurality of pages of the MLC flash memory are therefore divided into a plurality of pair pages, and each pair page comprises a strong page, a weak page, and a very weak page, wherein the strong page comprises the least significant bits of a set of memory cells, the weak page comprises the center significant bits of the same set of memory cells, and the very weak page comprises the most significant bits of the same set of memory cells. Because the strong page, the weak page, and the very weak page of a pair page store data in a single set of memory cells, the data content stored in the strong page affects the data content stored in the corresponding weak page and very weak page, and the data content stored in the weak page and the very weak page affects the data content stored in the corresponding strong page.
Referring to FIG. 1, a schematic diagram of a plurality of pair pages of a block of a TLC flash memory is shown. The block of the TLC flash memory comprises 192 pages with page numbers ranging from 0˜191, wherein every three pages form a pair page, and the block, therefore, comprises 64 pair pages. A first pair page comprises pages with page numbers 0, 4, and 10, wherein the page 0 is a strong page, the page 4 is a weak page, and the page 10 is a very weak page. A second pair page comprises pages with page numbers 1, 5, and 11, wherein the page 1 is a strong page, the page 5 is a weak page, and the page 11 is a very weak page. A third pair page comprises pages with page numbers 2, 8, and 16, wherein the page 2 is a strong page, the page 8 is a weak page, and the page 16 is a very weak page.
When data is stored in a flash memory, the data can only be read from the flash memory with a limited frequency threshold. If the data is read from the flash memory with a frequency greater than the frequency threshold, read-out data read from the flash memory would have a high error rate. The frequency threshold is referred to as data retention. For managing the data access of the flash memory, a controller stores important system data in the flash memory. To maintain the correctness of the important system data, the data retention of the system data must be extended. A data writing method capable of extending data retention of system data for a flash memory is therefore required.